Remember: A well-implemented first scan routine separates unreliable prototypes from industrial-grade automation. Take the time to initialize deliberately—your machine's safe operation depends on it.
fbFirstScan(); IF fbFirstScan.bFirstScan THEN // One-time initialization code here bInitDone := TRUE; END_IF
PROGRAM MAIN VAR bFirstScan AT %Q* : BOOL; // Not directly. Better: fbFirstScan : F_TRIG; bInit : BOOL := TRUE; END_VAR
Requires a couple of lines of code rather than referencing a direct tag. 🛠️ Method 2: The Variable Initialization Method beckhoff first scan bit
In the world of industrial automation, the moment a PLC (Programmable Logic Controller) transitions from "Stop" to "Run" is fraught with both opportunity and danger. Uninitialized variables, rogue previous states, and half-configured hardware can lead to catastrophic machine behavior.
| Test Scenario | Method | What to Verify | |---|---|---| | | Power cycle the controller | First scan code runs exactly once | | Warm Start | STOP → RUN from development environment | Custom flags execute if needed | | Program Download | Download without resetting retain variables | Retain-based initialization behavior | | Online Change | Modify code while running | New instances get proper initialization | | Multiple Power Cycles | Cycle power repeatedly | Counters increment correctly |
Edge detection using task cycle counter: Better: fbFirstScan : F_TRIG; bInit : BOOL :=
: Triggering a TP (Timer Pulse) or R_TRIG that needs to fire immediately upon startup.
VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Function block to get current task index bFirstScan : BOOL; END_VAR fbGetCurTaskIdx(); // Call the FB to refresh current task info bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; IF bFirstScan THEN // Logic here only runs on the very first PLC scan // e.g., Initializing setpoints or resetting state machines END_IF Use code with caution. Copied to clipboard 2. Manual Global Variable Flag
Initialization block with handshake:
The first scan is the perfect place to set default values, clear buffers, or initialize complex data structures that are not retentive.
Typical uses and patterns
This ensures a clean, new log file is created each time the controller starts, without overwriting historical data. | Test Scenario | Method | What to
VAR_GLOBAL bIsFirstScan : BOOL := TRUE; // Starts TRUE when the PLC runtime begins END_VAR Use code with caution. Copied to clipboard